1. Field of the Invention
This invention relates to the construction of a light receiving element and a manufacturing method of the semiconductor device including the light receiving elements.
2. Description of the Related Art
FIG. 1 shows the construction of a related art of a photodiode and a bipolar transistor which are light receiving elements and formed on the same substrate. In FIG. 1, the left area shows an example of the construction of the photodiode, and the right area shows an example of the construction of the bipolar transistor. An N type deposit layer 14 and a P type deposit layer P.sup.+ 15 are formed on an n type semiconductor substrate 11, respectively. Then a P type semiconductor element isolation region 17 is formed by growing an N type epitaxial layer 16.
A base region 18 of a P type semiconductor is formed at the right area on the epitaxial layer 16 and an N.sup.+ type diffusion layer 19 are formed in the left area and emitter and collector regions in the right area.
Nextly, an electrode layer of photodiode 22 and electrode layers of a base, an emitter and a collector of bipolar transistor 22 are formed in pattern. Then an insulation film is formed thereon. The thickness of the N.sup.- type epitaxial layer 16 is determined by the characteristic of bipolar element, and generally in the bipolar IC having approximately 10 [V] resisting power, the film thickness of 3-4 [.mu.m] was necessary.
According to the construction described above, the light receiving sensitivity of the photodiode is determined by the number of carriers generated in a vacant layer 100 and the number of carriers reached to the vacant layer 100 by diffusion out of carriers generated in the area interior to the vacant layer. Accordingly, in order to improve the light receiving sensitivity it was necessary to widen the vacant layer 100 and to increase the number of carriers to be brought in the vacant layer by providing semiconductor layer having long diffusion length at the upper and lower parts of the vacant layer 100. These two processes, to enlarge the width of vacant layer 100 and to provide the semiconductor layer having long diffusion length lead to control the impurity concentration.
On the other hand, the frequency characteristic of the photodiode is determined by the parasitic capacitance and parasitic resistance of the diode. Accordingly, in order to improve the frequency characteristic, it is necessary to decrease the parasitic capacitance and the parasitic resistance. In order to decrease the parasitic capacitance it is effective to enlarge the vacant layer 1000 and this means the decreasing of the impurity concentration of the junction. Furthermore, to decrease the parasitic resistance means to increase the impurity concentration of the semiconductor layer except the vacant layer. For example, referring to FIG. 1, the P type deposit layer 15 is provided to decrease the parasitic resistance of anode. Since the P type deposit layer 15 has high impurity concentration, the lengths of diffusion of a small number of carriers are short and the carriers which contribute to the light receiving sensitivity of the photodiode are almost all carriers generated at the upper part of this P type deposit layer 15.
These photodiodes are widely used to read information recorded on such as the compact disc (CD) and mini disc (MD). However, the wavelength of the semiconductor laser used in this type of optical disc is 780 [nm] and since the absorption length in Si of the laser with 780 [nm] wavelength is 9 [.mu.m], it created a problem that sufficient light receiving sensitivity could not be obtained due to the existence of P type deposit layer 15 positioned at 3-4 [.mu.m] from the surface in the conventional composition shown in FIG. 1.